New Zealand earthquake: Hundreds of aftershocks pepper North and South islands

A map from GeoNet showing the distribution of aftershocks. Photo: GeoNet​The deadly earthquake that struck 90 kilometres north of Christchurch on Sunday night has been followed by hundreds of aftershocks.

GeoNet, which monitors seismic activity in New Zealand, said: “We can say one thing with certainty: there will be more earthquakes to come in this area.”

About 10 hours after the huge quake hit, GeoNet had recorded 232 aftershock events. One of those tremors was of 6.1 magnitude. By noon AEDT close to 300 shocks had been recorded.

Caroline Little, a spokeswoman for GeoNet, told Fairfax Media that the agency was “pretty confident” the initial earthquake was two seismic events about 50 kilometres apart.

It is the largest earthquake recorded in New Zealand since 2009.

A statement from GeoNet said: “It looks like we’ve got two separate but related quakes going on. Our reports indicate that the combination of these two quakes lasted two minutes, with the most severe shaking at around 50 seconds.”

“It’s quite a complex quake,” Ms Little said. “It looks like two events happening almost simultaneously.”The M7.5 looks like TWO separate earthquakes, which is why we are seeing aftershocks in two locations. More info to come. #eqnz— GeoNet (@geonet) November 13, 2016

Geoscience Australia said that the seismic event released more than 50 times the amount of energy than the 2011 Christchurch earthquake that killed 185 people.

Dan Jaksa is the senior duty seismologist for Geoscience Australia. He said it was unlikely there were two separate quakes but that, as the earth’s crust ripped, seismologists can record movement at different locations.

“We put a dot on a map and call it the epicentre and give it a number but of course it’s more complex than that,” he said.

Mr Jaksa said an earthquake of this size had an impact zone of about 150 to 180 kilometres.

“What is occurring is a rip through the earth’s crust,” he said. “From our measurements it seems this one started onshore about 15 kilometres underground and ruptured the crust from west to east in a slightly north-easterly direction.”

GeoNet released an initial report of expected likely scenarios.Scenario 1 (Very likely): a normal aftershock sequence over the next few months. This means gradually diminishing size and frequency of seismic events.Scenario 2 (Likely): Possible rupture earthquake of magnitude 6 in North Canterbury and/or offshore.Scenario 3 (Unlikely): Another large earthquake above magnitude 7 within a month in the Marlborough and Cook Strait regions.

GeoNet measured the quake at a magnitude of 7.5, although the United States Geological Survey said it was of a 7.8 magnitude and Geoscience Australia said the event was of a 7.9 magnitude.

“There are a number of ways we can look at an earthquake,” Mr Jaksa said.

He said GeoNet relied on measuring close readings, which can “saturate” the measurement devices causing what he called “clipping”.

The first measurements from an earthquake are the “p-waves”, which are compression waves travelling through the earth, much like sound waves. Mr Jaksa said these dispersion waves allow detection of a quake’s location and an initial estimate of its size.

Following the “p-waves” seismologists measure “s-waves”, which are slower moving shear waves travelling perpendicular to the direction of the p-waves.

Seismologists then measure even slower surface waves that shake the crust in rolling waves allowing a better understanding of the event.

The different seismic waves and the way they travel across the earth: p-waves, s-waves, surface waves. Graphic: UNC Charlotte

Mr Jaksa said GeoNet relied on more localised measurements, whereas Geoscience Australia incorporated more longer-period waves. USGS uses a similar method, explaining why the US and Australian agencies have issued similar magnitudes for the event.

He said the NZ measurement was more like the older Richter scale, which measured amplitudes of seismic waves. The current scales incorporate measurements of earth movement.

Ms Little said that GeoNet’s use of local stations gave it a better ability to ﻿locate the epicentre and determine whether it was one or two seismic events.

“We’re pretty confident it was two,” she said.Last 48 hours in NZ in 30 seconds. #eqnz

Mr Jaksa said: “These scales are logarithmic, which means each unit increase represents a 10 times increase in size.

“However, this doesn’t account for the amount of energy released. The event overnight released 50 times the amount of energy than the 6.1 magnitude Christchurch quake did in 2011.”

That quake was just five kilometres underground and 10 kilometres from the city centre. It killed 185 people.

Mr Jaksa said: “This is the most active seismic site on earth. It stretches from the Solomon Islands through Vanuatu and down to New Zealand.

“The Pacific Plate is going under the Australian Plate along this area. The Pacific Plate is moving 11 centimetres a year in a west-north-west direction. The Australian Plate is moving north-east at about seven centimetres a year,” he said.​

However, along the South Island things get complicated. Here the Australian Plate is largely going under the Pacific Plate.